1. Field of the Invention
The present invention relates to photoalignment materials and to their use in liquid crystal displays.
2. Description of the Related Art
Flat panel display devices, including liquid crystal display devices, are widely used in cellular phones, notebook computers, and wide screen televisions. Advantages of flat panel displays include low power consumption, lightweight, thinness, and portability.
A liquid crystal display device includes a liquid crystal panel. Such panels are formed by aligning two prepared substrates at a constant interval, and then injecting a liquid crystal between them. To serve as a display device, such liquid crystal display devices should meet the requirements of good light transmission, fast response time, wide viewing angle, and good contrast.
Because those requirements are dependent on the alignment characteristics of the liquid crystal molecules in the liquid crystal panel, it is important to properly align those molecules. However, proper alignment is not accomplished by simply injecting liquid crystal molecules between two substrates. An alignment film is generally required.
A polyimide resin is usually used for the alignment film. The chemical structure of polyimide can be divided into a main chain, which plays a major role for aligning liquid crystal molecules in one direction, and a side branch, which forms a pretilt angle.
To form a liquid crystal alignment film using polyimide, the polyimide is normally coated as a thin alignment film (with a thickness of 1 micrometer or less) and dried. Then, the surface of the thin film is physically rubbed with a specifically manufactured cloth. This process is typically used when manufacturing liquid crystal displays because it is simple and easy.
Unfortunately, rubbing is not appropriate for alignments over large areas. Furthermore, the many variables associated with the process are difficult to control. Also, an alignment film can be contaminated by fine particles or fibers produced by the rubbing process, and thin film transistors used in the display can be damaged by static electricity produced by rubbing. These problems can result in low quality or defective devices.
New methods to solve the foregoing problems are being developed. For example, photoalignment methods might enable simple and easy to align polymer surfaces without rubbing.
Known photoalignment materials include polyvinyl-cinnamates and polyvinylmethoxy-cinnamate. Those polymers produce photoalignment, but problems exist with long term alignment stability and thermal stability. Besides, those materials generally establish low pretilt angles. Thus it is difficult or impossible to obtain a desired pretilt angle for the liquid crystal alignment.
Thermal issues are significant since an alignment film's stability is dependent on the thermal stability of the polymer per se. In particular, photoalignment materials having a glass transition temperature below 100° C. are not appropriate for alignment layers since thermal stability around the glass transition temperature is poor.
Although many attempts have been made to solve the problems, a new photoalignment material that is sensitive to ultraviolet light would be beneficial.